The use of sound in animation and film has always played a vital role, and moreso in films of little dialogue, such as Wall-E to help convey emotion, stage a scene and tell the story. Ben Burtt is considered the father of modern sound design, having first started work on such films as the original Star Wars, and more recently, Wall-E. Whereas Disney had always used instruments for sound effects in its animations, such as a cymbal crash for a collision, Burtt starting improvising and creating his own "instruments" out of whatever made the most believable sounds, from slinky springs to pieces of fruit. This is because, as with the drawn designs and movements in animation,

Believability is more Important than Realism

That is to say, that the emotion a sound evokes, if the correct emotion, is more important than if it is the correct physical sound being produced. For example, is a spring being struck sounds like a lazer being fired, then that it what should be used rather than the realism of a lazer being silent. This is because theKercheeww orZzzap feels more emotionally right for something we are unfamiliar with, and is then replaced in our minds that this is what these things should sound like.

Modern techniques for voice manipulation, control instruments and digital synthesizers have widened the field of sounds that can be produced but are no good unless used in the right settings, again all about believability.

I think care must be taken with sound, as people can easily pick up on sounds that are out of place or just "not right", especially in films that are more based around physical expression than dialogue.